The userid in question has access to R(UNI) $KEY(SUPERUSER) "FILESYS.-" with SERVICE(READ) ALLOWAs per IBM documentation this is what should be required (as it was working fine with z/OS 2.2):SUPERUSER.FILESYS.MOUNT on the UNIXPRIV class.

Answer:

In z/OS 2.3, the mount is done with SETUID and requires UPDATE authority instead of READ.